fixdep: remove multiple .config support code

Since commit e02ee254 (kconfig: switch to single .config
configuration), the ".*.cmd" files are not correctly created
for SPL/TPL.  The U-Boot extension code in fixdep, which was
introduced to support the multiple .config, must be removed.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>